With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using magnetic recording technology is continuously increasing at an annual rate of approximately 100%. In recent years, an information recording capacity exceeding 200 gigabytes per perpendicular magnetic recording medium with a 2.5-inch diameter for use in an HDD or the like has been desired. To fulfill such demands, an information recording density exceeding 400 gigabits/inch square is desired to be achieved.
To attain a high recording density in a magnetic recording medium for use in an HDD or the like, a perpendicular magnetic recording type has been suggested in recent years. In a perpendicular magnetic recording medium for use in the perpendicular magnetic recording type, the axis of easy magnetization of a magnetic recording layer is adjusted so as to be oriented in a direction perpendicular to the base surface. In the perpendicular magnetic recording type, compared with a conventional in-plane recording type, it is possible to more suppress a so-called thermal fluctuation phenomenon, in which thermal stability of a recording signal is impaired because of a superparamagnetic phenomenon to cause the recording signal to be lost, and therefore the perpendicular magnetic recording type is suitable for increasing the recording density.
As a magnetic recording medium for use in the perpendicular magnetic recording type, a CoCrPt—SiO2 perpendicular magnetic recording medium (refer to Non-Patent Document 1) has been suggested because of high thermal stability and excellent recording characteristic. This is to configure a granular structure in a magnetic recording layer in which a non-magnetic grain boundary part with segregation of SiO2 is formed between magnetic particles in which a crystal with an hcp structure (a hexagonal close-packed crystal lattice) of Co continuously grows in a columnar shape, thereby achieving finer magnetic particles and an improvement of a coercive force Hc together. As the non-magnetic grain boundary (a non-magnetic portion between magnetic particles), using an oxide has been known and, for example, using any one of SiO2, Cr2O3, TiO, TiO2, and Ta2O5 has been suggested (Patent Document 1).
In the magnetic recording medium with such increased recording density as described above, a further improvement in recording density is demanded for the future. Various important factors for increasing recording density of the perpendicular magnetic recording medium include an improvement in magnetostatic characteristic, such as a coercive force Hc and a reversed magnetic domain nucleation magnetic field Hc; an improvement in electromagnetic conversion characteristic, such as an overwrite characteristic (OW characteristic) and an SNR (Signal to Noise Ratio); and narrowing of a track width. Among all, an improvement in SNR is important for reading and writing accurately at high speed even at a recording bit of a small area.
An improvement in SNR is achieved mainly by reducing noise in a magnetization transition region of the magnetic recording layer. Effective factors for reducing noise include an improvement in crystal orientation of the magnetic recording layer, making a finer particle diameter of each magnetic particle, and isolation of the magnetic particles. Among others, when isolation of magnetic particles are promoted, an exchange interaction is interrupted. Therefore, noise can be greatly reduced and the SNR can be significantly improved.
To this end, providing a ground layer under the magnetic recording layer has been tried. The ground layer has an hcp structure, and has an operation of growing a crystal with an hcp structure of the magnetic recording layer as a granular structure. Therefore, the orientation of the magnetic recording layer can be improved as the crystal orientation of the ground layer is higher, that is, a (0001) surface of the crystal of the ground layer is more parallel to the main surface of a disk base. A typical material of the ground layer is Ru.